Relation pm#syntax (?,description_content/medium/container)
  exclusion:  attributive_relation  purpose  has-author  owned-by  owner  owner_of  leader  seller  customer  generator  creator  spatial_origin  exploits  has_purpose  has_purpose_for_agent  measure  attribute  name  Date  value  rdf_reification_relation  support  example  has-magnitude
  supertype:  attributive_relation__attributiverelation (*)  like pm#binary_relation, this type mostly exists to categorize what cannot be categorized elsewhere
     supertype:  relation_playing_a_special_role (*)  this type permits to categorize relations according to their roles ; this is a traditional but quite subjective way of categorizing relations
        >part of:  relation__related_thing__relatedthing___related_with  type for any relation (unary, binary, ..., *-ary) and instance of pm#relation_type